A typical computer contains a number of different types of memories for storing data. For example, an electronic random access memory (RAM) is normally used as the main working memory for the computer while it is operating. Typically, a dynamic random access memory (DRAM) is used for this purpose. This type of memory is volatile, which means that any data stored therein is lost as soon as the power to the memory disappears, e.g. the computer is turned off. For permanent storage of data, a computer typically includes various forms of non-electronic memory, e.g. magnetic and/or optical disks.
In addition to these two primary forms of memory, a computer may also include smaller, non-volatile electronic memories for storing persistent information that pertains to the operation of the computer. In the context of the present invention, "persistent" data is data that must continue to be stored in the computer after the computer is turned off, and thereby be available each subsequent time the computer is turned on. For example, the computer may include a read-only memory (ROM), which functions as a control store that contains microprograms for the computer's instructions, as well as startup and interrupt processing information. Since this type of memory has read-only capabilities, it is not suited for storing variable information. However, it may be desirable to use an electronic memory to store persistent data that is changeable. For example, a user may configure a computer with certain desired operating preferences that remain in effect each time the computer is used. While it is possible to store this type of information on a magnetic or optical disk, and load it into the computer's random access memory each time the computer starts, it is preferable to store these parameters in an electronic random access memory.
In order for a random access memory to effectively store persistent data, the memory must be provided with a continuous supply of power to preserve the contents of the data stored therein. One type of memory that functions in this manner is a non-volatile memory. Typically, non-volatile random access memories have been available as self-contained devices, which include an integral battery that functions as the source of continuous power. Since the non-volatile RAM is incorporated within the structure of the computer, it is desirable that the battery which supplies power to it have as long a life as possible, to thereby minimize the inconvenience of having to replace the battery. Accordingly, non-volatile random access memories which include an integral power supply have employed lithium batteries, because of their long life. Unfortunately, lithium batteries are relatively expensive, which means that the overall cost of a non-volatile random access memory can be quite high. Furthermore, since the battery is integral with the memory, when the battery's power has been consumed, the entire non-volatile memory device must be replaced, which further adds to the overall cost to the consumer.
Another type of electronic memory device that has been employed in the past to store persistent data is a flash EEPROM. Unfortunately, these types of devices are relatively expensive, in part due to the fact that they are typically designed to store large amounts of information, and therefore are not cost-justifiable for use as a parameter RAM or in other situations where only relatively small amounts of persistent data, e.g., less than 8K bytes, needs to be stored. In addition, they require a complex interface for reading and writing data, which further adds to their expense.
In addition to the non-volatile RAM, modern computers include other components which require a constant source of power. For example, the computer might include a real-time clock chip which maintains the proper date and time of day, even when the computer is turned off. Some computers may also include a microcontroller that controls initial power-up of the computer, and is therefore kept constantly operational by a battery. Since each of these components also requires a continuous source of power, some computers may include a battery which is separate from the battery incorporated into the non-volatile random access memory. The presence of two or more batteries in a computer, one of which is dedicated to non-volatile random access memory, and another which powers other components, results in a concomitant increase in the overall price of the computer, particularly when relatively expensive batteries, such as lithium batteries, are employed.
It is desirable, therefore, to provide an arrangement in which a single battery can be utilized as the source of power for all non-volatile components in a computer, including a RAM for storing persistent data. In such a case, a less expensive memory device, which does not include an integral battery, can be employed for persistent data storage.